1. Field of the Invention
The present invention relates to a polymer-dispersed reflective polarizer, and more particularly, to a polymer-dispersed reflective polarizer in which a light modulation effect can be maximized using a smaller number of polymer components disposed inside a matrix.
2. Description of Related Art
Flat display technologies cover, as main items, liquid crystal displays (LCDs), projection displays and plasma display panels (PDPs), which already occupy a significant share of the market. Field emission displays (FEDs) and electroluminescent displays (ELDs) are also expected to occupy their own market share, based on their characteristics in response to the advancement of related technologies. LCDs are expanding their application range over notebook computers, personal computer (PC) monitors, liquid crystal TVs, automobiles, aircraft, and the like, and occupy about 80% of the flat display market. The LCD industry has enjoyed prosperous times to date because of the rapid increase in the worldwide demand for LCDs.
In LCDs in the related art, a liquid crystal and an electrode matrix are disposed between a pair of light-absorbing optical films. In LCDs, liquid crystals change their orientation in response to an electric field that is generated by applying a voltage to two electrodes, thereby changing the optical properties thereof. This processing displays an image in “pixel” units, which contain information, using polarization in a specific orientation. Because of this, LCDs include a front optical film and a rear optical film, which induce polarization.
Optical films used in such LCDs cannot be said to use light projected from a backlight with high efficiency. This is because at least 50% of the light projected from the backlight is absorbed by a rear-side optical film (an absorbing polarizer film). Therefore, in LCDs, a reflective polarizer is disposed between an optical cavity and a liquid crystal assembly in order to increase the efficiency with which backlight light is used.
FIG. 1 is a view showing an optical principle of a reflective polarizer of the related art. Specifically, P polarization of light that is directed from the optical cavity to the liquid crystal assembly is allowed to pass through the reflective polarizer so that it is forwarded to the liquid crystal assembly. S polarization of light that is reflected from the reflective polarizer to the optical cavity is reflected from a diffusion-reflecting surface of the optical cavity in the state in which the direction of the polarization is randomized, and is then forwarded to the reflective polarizer again. Finally, the S polarization is converted into the P polarization, which can pass through the polarizer of the liquid crystal assembly. Consequently, the converted P polarization passes through the reflective polarizer, so that it is forwarded to the liquid crystal assembly.
This processing of the reflective polarizer on incident light, in which the S polarization is selectively reflected and the P polarization is allowed to pass through, is established due to the setting of the optical thickness of each optical layer and variation in the refractive index of each optical layer, which are caused by the difference in refractive indexes between the optical layers and the stretching of the stacked optical layers in the state in which flat optical layers having an anisotropic refractive index and flat optical layers having an isotropic refractive index are stacked such that they alternate with each other.
That is, light incident on the reflective polarizer is repeatedly reflected as S polarization and is allowed to pass through as P polarization while passing through respective optical layers, so that only the P polarization of the incident light is finally forwarded to the liquid crystal assembly. On the other hand, as described above, the reflected S polarization is reflected from the diffusion-reflecting surface of the optical cavity in the state in which the polarized state is randomized, and is then forwarded to the reflective polarizer. This makes it possible to reduce the loss of light that is generated from a light source and the consequent waste of electrical power.
However, the reflective polarizer of the related art has a problem in that its manufacturing process is complicated, since the reflective polarizer is manufactured by stacking the flat isotropic optical layers and the flat anisotropic optical layers, which have different refractive indexes, such that they alternate with each other, and by stretching the stack of the layers such that the optical thickness and refractive index of each optical layer are optimized to selectively reflect and transmit incident polarization. In particular, since each optical layer of the reflective polarizer has a flat panel structure, P polarization and S polarization must be divided in response to the wide range of the angle of incidence of incident polarization. Therefore, the number of optical layers that are stacked is increased excessively, thereby exponentially increasing the production cost. In addition, the structure in which the number of the stacked optical layers is excessive causes a problem in that light loss may degrade the optical performance thereof. In addition, in the related art, when a skin layer made of polycarbonate and a core layer, which is a stack of alternating PEN/co-PEN layers, are integrated together via co-extrusion, peeling may occur due to the absence of compatibility. Because the degree of crystallinity is about 15%, the danger of birefringence with respect to the stretching axis is high when stretching is performed. Therefore, a bonding layer must be formed between the core layer and the skin layer in order to apply a polycarbonate sheet without stretching. As a result, the addition of the bonding layer results in a decrease in yield due to the presence of foreign impurities and the incidence of process defects. Typically, when an unstretched polycarbonate sheet of the skin layer is produced, birefringence occurs due to the irregular shearing pressure caused by the winding process. In order to correct this, additional processing, such as modification of the molecular structure of polymer and control over the speed of an extrusion line, is required, thereby lowering productivity.
Accordingly, disclosed is a technical concept that can realize the function of a reflective polarizer by arranging a birefringent polymer, which is stretched in the lengthwise direction, inside a matrix. FIG. 2 is a perspective view of a reflective polarizer 20 that contains a rod-like polymer. Inside a matrix 21, a birefringent polymer 22, which is stretched in the lengthwise direction, is arranged in one direction. Then, a light modulation effect is induced by the birefringent interface between the matrix 21 and the birefringent polymer 22, so that the function of a reflective polarizer can be performed. However, there is a problem in that the light modulation efficiency is too low compared to the foregoing reflective polarizer, which comprises a stack of alternating layers. In order to have transmittance and reflectivity similar to those of the reflective polarizer having the stack of alternating layers, an excessive number of rods of birefringent polymer 22 must be arranged inside the matrix, which is problematic. Specifically, when manufacturing a display panel having a width of 32 inches with respect to the vertical cross-section of the reflective polarizer, at least one hundred million spherical or oval particles of the birefringent polymer 22, the cross-sectional diameter of which ranges from 0.1 μm to 0.3 μm in the lengthwise direction, must be contained inside the matrix 21, which has a width of 1580 mm and a height (thickness) of 400 μm or less, in order to achieve optical properties similar to those of the stack-type reflective polarizer. In this case, the production cost is excessively increased, facilities are excessively complicated, and it is almost impossible to manufacture a spinneret, which produces the polymer. Therefore, it is difficult to commercially distribute this reflective polarizer.
In order to overcome such problems, a technical concept in which birefringent sea-island fibers are contained inside a matrix was proposed. FIG. 3 is a cross-sectional view of a birefringent sea-island fiber contained inside the matrix. The birefringent sea-island fibers may create a light modulation effect at the light modulation interface between the inside island part and the sea part. Unlike the foregoing birefringent polymer, desirable optical properties can be realized even if a large number of sea-island fibers are not arranged. However, since the birefringent sea-island fibers are fiber, some problems, such as compatibility with the polymer matrix, ease of handling, and adherence, occur. Furthermore, the circular configuration induces the scattering of light, so that the efficiency of reflective polarization of light wavelengths in the visible light range is reduced. Polarization characteristics are degraded compared to existing products, and thus the improvement in illumination is limited. In addition, sea-island fibers suffer from degradation in light characteristics due to light leak or light loss, since the junction between the islands is reduced but the sea area is subdivided, thereby forming gaps. Furthermore, the texture-like tissue structure limits the construction of layers, so that the improvement in the reflection and polarization characteristics is limited.
The information disclosed in this Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.